[LLDB][ClangExpression] Remove unused StructVars::m_object_pointer_type

This member variable was removed a while ago in
44342735. It was previously used in
materialization code paths that have since been removed. Nowadays,
`m_object_pointer_type` gets set but not used anywhere.

This patch simply removes all remaining instances of it and any
supporting code.

**Testing**

* API tests pass

Differential Revision: https://reviews.llvm.org/D129367